Phosphorescent organic light emitting diodes (OLEDs) have attracted considerable commercial and academic interest for application in high-quality, energy efficient flat-panel displays. Complexes based on cyclometallated second and third row transition metal centres (in particular iridium and platinum) exhibit highly efficient phosphorescence and have commonly been incorporated in these devices. OLEDs operate on the principle of electroluminescence. Typical OLEDs consist of several layers which are sandwiched between electrodes (Tang, et al., Appl. Phys. Lett. 1987, 51, 913). Upon applying an electric field, holes are injected from the anode and electrons are injected from the cathode of the OLED and migrate towards each other. When the charge carriers meet, they recombine with the emission of light of a certain wavelength according to the nature of the emitter used. The recombination process creates both singlet (25% probability) and triplet (75% probability) excitons. In simple organic molecules triplet excitons do not effectively produce emission of light and, therefore, the theoretical quantum efficiency is limited to approximately 25%. However, the introduction of metal centres like platinum and iridium, facilitates inter-system crossing between singlet and triplet states and promotes efficient radiative decay from triplet states to a ground state. Thus the use of cyclometallated second and third row transition metal centres in OLEDs allows the harvesting of both singlet and triplet type excitons therefore increasing the maximum obtainable internal quantum efficiency of the device. The introduction of two or more metal centres may further facilitate inter-system crossing between singlet and triplet states and thus may further increase efficiency.
In addition, complexes of this type are known to have further applications such as light emitting units of sensory molecular systems whose emission is modulated upon binding of a target analyte or as intrinsically emissive probes that may localize in selected organelles in living cells. Furthermore, they are under investigation as potential photocatalysts for “water splitting” to generate hydrogen, as sensitizers of energy and electron-transfer reactions and are also relevant to processes involving the conversion of solar energy to electrical energy. For these applications efficient absorption and emission in the red region of the electromagnetic spectrum is desired. Introducing a second metal centre is one way of shifting absorption and emission into the red without the need to extend the conjugation system of the ligand and potentially minimizes non-radiative decay pathways associated with band gap law which would lead to a decrease in quantum efficiency.
To date a variety of highly luminescent cyclometallated complexes with transition metal centres have been synthesized and many have been used in OLEDs and the other technological fields mentioned above. Typically such compounds contain a single metal centre surrounded by a number of ligands which bind to the metal via one or more donor atoms. Examples of cyclometallated complexes with one metal centre are disclosed in Brulatti et al., Inorganic Chemistry, 2012, 51(6), 3813-3826, wherein a series of monoirdium complexes based on tridentate NCN ligands are described. Luminescent cyclometallated complexes containing two metal centres have also been synthesized. Examples include bidentate ligands including di-iridium cyclometallated complexes (Tsuboyama et al., Dalton Transactions, 2004, (8), 1115-1116), platinum and iridium mixed metal complexes (Kozhevnikov et al., Inorganic Chemistry, 2011, 50(13), 6304-6313) and bimetallic complexes incorporating bidentate carbine ligands (Tennyson et al., Inorganic Chemistry, 2009, 48(14), 6924-6933). However, octahedral complexes formed from such ligands are often chiral and when two or more metal centres are present in one molecule complications can arise due to the formation of mixtures of diastereomers. As diastereomers have different chemical and physical properties characterisation of diastereomeric mixtures is very difficult and because the physical properties of the compounds contained therein are not uniform, application in OLEDs is compromised. To enhance the suitability of cyclometallated complexes for OLEDs it is therefore highly desirable that the diastereomers can be separated. Another approach is to use nonchiral metal centres. However, very limited number of nonchiral multimetallic cyclometallated complexes have been investigated to date and none have demonstrated high luminescent yields for application to OLEDs. For example, di-ruthenium complexes coordinated by bis NCN cyclometallated ligands wherein the metal centres are connected by a flexible linker are known (Yang et al., Organometallics 2012). In addition, cyclometallated complexes with mixed metal centres held by cyclometallating terdentate ligands have been also synthesized (Wu et al., Organometallics 2012, 31(3), 1161-1167).
Despite the existence of multimetallic cyclometallated metal complexes, their full potential has yet to be realized. What is therefore needed are new highly luminescent multimetallic complexes which are easily synthesised and can deliver high luminescent quantum yields and be applied to OLEDs and other technology fields.